Device for ejection of moulded pieces from a moulding tool

ABSTRACT

The invention relates to a so-called spring cushion ( 5 ), for the ejection of moulded pieces ( 4 ) from a moulding tool ( 3 ), with a plunger ( 6 ), moving in a guide tube ( 9 ) within the moulding tool and loaded in the ejection direction ( 14 ) by means of a spring ( 15 ). According to the invention, the spring ( 15 ) is embodied as a draw spring, in particular as a spiral draw spring. Considerably longer service life can be achieved as draw springs do not have radial deflections on loading.

TECHNICAL FIELD

The present invention relates to a device for ejecting formed parts, inparticular those made of sheet metal, from a forming die, having aplunger movably guided in the forming die in a guide tube and loaded inthe ejection direction by a spring.

PRIOR ART

Such devices are known and are also designated as spring cushions. Thesprings used in such spring cushions are generally compression springs.

The forming die may be, for example, a multistage compound die,consisting of a top part and a bottom part, in a press. In this case,the top part is fitted into the press ram and performs stroke movementstogether with the latter. Modern presses run with high stroke rates ofseveral hundred strokes per minute. Between each stroke, the parts to beformed must be reliably ejected, for example, from the top part.Production figures of 100 million pieces per year are not uncommon.Under such conditions, it has been possible hitherto to achieve a toollife of about ⅓ year on average. In this case, the wear or the failureof the spring cushions has proved to be a limiting factor.

DESCRIPTION OF THE INVENTION

The object of the invention, inter alia, is to specify a device forejecting formed parts from a forming die, having a plunger movablyguided in the forming die in a guide tube and loaded in the ejectiondirection by a spring, this device being distinguished by a longerservice life. According to the invention, this object is achieved insuch a device by a tension spring being used as the spring.

The invention is based on the knowledge that the compression springsused hitherto in the spring cushions tend to deflect radially as soon asthey are compressed and that the deflection can become so large that thespring touches the guide tube. This results in metallic abrasion andultimately leads to the failure of the spring cushion. By a tensionspring being used according to the invention instead of a compressionspring, this problem is advantageously avoided and a longer service lifeis achieved in the sense of the object specified.

According to a first preferred embodiment of the invention, a helicalspring is used as the tension spring and in such a way that the guidetube and the plunger are arranged essentially inside this helicalspring.

According to a further preferred embodiment, to attach the spring to theguide tube and to the plunger, screw plugs are provided in each case onthe guide tube and on the plunger, into which screw plugs are screwedinto the spring ends.

The tension spring is extended under loading. However, the spring wire,with regard to its pitch, is fixed on the two screw plugs. In thetransition region from the screw plugs to the free region, this resultsin a jump in pitch and stress under loading, with unequal loading of thespring wire in torsion. In order to alleviate this nonuniformity of thespring loading and prolong the life of the spring, provision is madeaccording to yet a further embodiment of the invention for the pitch ofthe thread on the screw plugs to be dimensioned in accordance with anaverage spring pitch, the expression “average spring pitch” referring tothe pitch of the spring in a center position of the plunger between itstwo end positions.

With spring cushions according to the present invention, it has beenpossible to achieve a tool life of at least one year at said productionrates.

BRIEF EXPLANATION OF THE FIGURES

The invention is to be explained in more detail below with reference toexemplary embodiments in connection with the drawing. The drawing showsa section through a spring cushion according to the invention in thefitted state.

WAYS OF IMPLEMENTING THE INVENTION

In the drawing, 1 designates the ram of a press, which itself is notshown. The top part, fastened to the ram 1, of a forming die isdesignated by 2, the actual forming die is designated by 3, an alreadyformed sheet-metal part is designated by 4, and a spring cushionaccording to the invention inserted into a stepped bore in the press ram1 is designated by 5.

The spring cushion 5 in turn comprises a plunger 6 which is displaceablein its longitudinal direction in two sliding guides 7 and 8. The slidingguides 7, 8 are arranged on the inside on a guide tube 9. The guide tube9 is part of a type of sleeve 10 which also has an outer tube 11. Theouter tube 11 leads into a flange 12 which is secured to a step 13 ofthe abovementioned stepped bore.

The plunger 6 of the spring cushion 5 is preloaded in the ejectiondirection, identified by arrow 14, by a helical tension spring 15 whichis directed around the plunger 6 on the outside. The top end section 16of the spring 15 is screwed onto a plug 17 and the bottom end section 18of the spring 15 is screwed onto the guide tube 9 onto or into theannular gap between guide tube 9 and outer tube 11 of the sleeve 10. Theguide tube 9 and the outer tube 11 or the sleeve 10 thus likewiseconstitute a type of bottom plug for the spring 15.

The drawing shows the spring cushion plunger 6 in its fully extendedbottom end position, in which the spring 15 is loaded to the smallestextent and therefore has only a small pitch along its center region,where it is free. If the plunger 6, during the closing of the die, ispressed upward by the spring travel F into its top end position, thespring 15 is loaded to the maximum extent and obtains the maximum pitchin its free region. Of course, in a plunger position at precisely halfthe distance between the two end positions, the spring 15 has an averagepitch. The spring 15 is secured to the two plugs by the thread. As aresult, the pitch of the spring 15 does not change here at thetransition between the two end positions. In order to preserve thespring and keep the difference in pitch between its center free sectionand its two end sections as small as possible on average, the pitch onthe end sections is selected to be approximately equal to theabovementioned average pitch.

LIST OF DESIGNATIONS

1 Ram of a press  2 Top part of a forming die  3 A forming die  4 Analready formed sheet-metal part  5 Spring cushion  6 Plunger 7, 8Sliding guides  9 Guide tube 10 Sleeve 11 Outer tube 12 Flange 13 Step14 Arrow 15 Helical tension spring 16 Top end section 17 Plug 18 Bottomend section

1. A device for ejecting formed parts from a forming die having a plunger movably guided in the forming die in a guide tube and loaded in the ejection direction by a spring wherein, the spring is a tension spring, and wherein the tension spring is a helical spring, wherein the guide tube and the plunger are arranged essentially inside this helical spring.
 2. The device as claimed in claim 1, wherein, to attach the spring to the guide tube and to the plunger, screw plugs are provided in each case on the guide tube and on the plunger, the screw plugs each being screwed into ends of the spring ends.
 3. The device as claimed in claim 2, wherein the spring, in a center position of the plunger, has an average pitch, and wherein the pitch of the thread on the screw plugs in each case corresponds to this average spring pitch. 